The IF section gain wanders around from 100uV to 700uV input for the same output. Not long after turning the radio on, it 'clicks' and then the sensitivity jumps way up (less uV for same output). Then as it warms up it wanders around. I've swapped the tubes and that doesn't make any difference. Ideas? I'm watching the output on a VM measuring the diode load, but I can also hear the noise go up and down with the diode load output.

Also does anyone know of an R390A specific forum or site? I found the R390A FAQ site, but there is no forum there that I see.

I have the receiver in MGC. That's the way it's tested through the IF amp, but I'm not saying it couldn't be a similar issue. It's just curious that when it 'snaps' the gain goes way up, but then it slowly goes down to wander around for a while and finally settles somewhere in the middle after a half hour or so. It sure acts like a tube, but undoubtedly it's a cap somewhere, maybe. I haven't replaced them yet, it's on the list. I'm getting a list up for mouser to order.

I haven't messed with the other IF strip, it has bad mechanical filters, but I've replaced the caps in it. I'll try it next.

It's also curious that the loss through different filters seem to change with time too, I suppose the loss through the filters could change with temp. I need to get the freeze spray out, but not on the tubes.

How about an old carbon comp. resistor that heats up and goes way out of spec? I would check or replace any high value resistors in the IF deck (most likely in the AGC line or screen dropping resistors as well.

Gud luck with it, sooner or later the solution will smack you in the forehead.

If the receiver gain is snapping up and down and moving any of those little coax connectors between modules doesn't make this happen, then it's going to to be a little work looking for plate resistors out of tolerance in the IF strip. And possibly coupling caps. So, you'll need a 'Scope to look at the 455 IF and see what stage is giving the unsteady RF level. Tubes can be troublesome too. Very lightly tap on the tubes to try to make the problem occur. And don't forget the 1st and 2nd oscillators on the RF deck. That was a problem for me.

The RF deck is out of the equation, I'm feeding the IF strip only with 455. Yes, I have caps coming, and I've switched tubes with an IF module that does not do it, no change. I've got some tube extenders and I need to start checking the voltages on the tubes as it happens. Trouble is it only does it for about a half hour, then I have to let things cool off to start over. I suppose I could lay out the IF strip with the RX on it's side and start shooting freeze spray on things. It may come to this.

N3DTCan I ask if you have the schematic? There may be certain RF levels you will have to inject into the I.F. I do not know the impedance 75/50 ohms? The R390A manual has a different approach to trouble shooting the different modules. On or around Pg 90 or 91 they give steps to check the gain for each stage in the receiver. They are using an older signal generator that I am not familiar with.But you'll have to make sure you are not overloading the strip from your source.You troubleshooting is pretty much on target and I know it gets frustrating when it is just for a half hour of messing up. Freezing and a hair dryer can "cycle the problem" so you can locate the stage or area.My luck has been that freezing and a hairdryer are too extreme or fast and the problem doesn't happen at a regular interval, so you can be readyKeep us informedFred

Yeah, I've got the Navships book. I have a Motorola Service monitor, so it's calibrated in dbm and uV too. All 50 ohm. The book says to feed 150uV into the IF section and set up the gain for -7VDC on the diode load which is what I start with. I've got new caps and some other stuff coming and hopefully this will fix it. Other than that, the whole rx is working pretty good, when the gain is right I've got sub 1uV sensitivity. I've got another IF strip that is good on the 8 and 16KHz filters, the rest are bad but it's a good check, plus the other strip has a bad AGC detector plate load Z so there's no AGC action on that one. Working on that too.

N3DTSlowly you reveal that you have all the right stuff. Moto service monitor. I had an IFR that I had to get rid of on eBay. And bought a Marconi. Big heavy clunker sig gen and the output is not calibrated. But a signal source.The 1uv is pretty good average over the bands. With the IFR gone I have no idea where my R390A is at these days.The AGC ran me around for about a week and turned out to be resistors in various stages that were out of tolerance. This was a module I sent to Rick Mish btw. I'm sure he does quality work, and maybe prefers the complete radio as to working on the modules.I also blocked any chance of a D.C. voltage getting to the precious filters with a .01 cap to pass the 455 I.F. in and out of the filters.Fair Radio is still a very good source of parts and goodies for the 390A. I'm tempted to get an RF deck for a spare.Fred

I haven't found any out of spec resistors yet. I was thinking C551 may be the culprit, but it's grounded out of the circuit in MGC. I got a bunch of metalized caps, but I'm thinking I may just go with ceramic for bypass and coupling. I did use the Orange Drops in the filter coupling caps to avoid the dc on the filters problem. I put the OD everywhere in one of the IF modules and now it's so crowded I can't work in there. Yeah, I'm looking at a Z503 at Fair Radio for $16. I could use some chokes for the PS (AF module) in one too, but I'm checking out some Hammond chokes, they are reasonably priced and they may even fit in the old cans. I pulled apart L601 yesterday and the thing was burned up, reading about 40 ohms. I'm talking with the guys at the R390 forum too.

Remove tube V503 and insert one lead of a 0.01 uF capacitor into pin 5. (Caution high voltage present at this plate pin). Connect the signal generator to the other lead of the capacitor and set the generator level for the same output level you used in previous tests. Repeat the drift test. If it still drifts, the problem is after V503. If the drift disappears, the problem is in front of V503 so reinsert V503, remove V502, insert the capacitor at pin 5, and repeat the test. Work your way back up to the input tube V501 until you find the drift occurs.

You can also perform this isolating test by inserting the signal generator at a tube grid instead of the plate. The tube must be in place so you have figure a way to route the signal generator signal under the IF subchassis.

I've got tube extenders with taps on the top. I made a DC block for the R2005 Service Monitor with a 1KV .01uF ceramic. The way I'd approach it is to send the signal to the grid of V503, that would likely cut out 1/2 of the amp sections right away, then half the bad side, etc. It probably wouldn't take long there's only 4 amps, but like I say, it takes a while for it to cool off and start over again. Fan would help. I've been messing with other things, but I need to get back on it. It would be nice to know exactly what it is instead of shotgunning the thing. I'll get it as long as the weather doesn't get too good and I need to go outside.

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